The invention relates to casings for the preservation and/or flavoring of processed foods, e.g. processed meats.
Foods are often processed, i.e. cooked, in a plastic film package by for example at least partially immersing the package in hot water or placing the package in a steam-heated environment. The processed food package may then be refrigerated until the processed food is to be consumed. During the meat cooking process (e.g. frankfurter or smoked turkey breast), excess water vaporizes though the casing to ambient while smoke or other modifiers for color and/or flavor are diffused through the casing into the meat.
The current standard industry food casing is made of cellophane in a tubular form. The starting material is generally wood-pulp. It is saturated in an 18% sodium hydroxide solution to swell the cellulose structure. The wood-pulp is subsequently reacted with carbon disulfide to form a paste, named xe2x80x9cviscosexe2x80x9d. The viscose is aged (or ripened) prior to the actual formation of the film. To produce the cellophane film, the first step is to extrude the viscose through a circular die. The tube shape viscose is then immersed in a 20 weight percent sodium sulfate and 2 weight percent sulfuric acid solution to coagulate the structure. The next step is to regenerate the coagulated tube by reacting the material with a 10 weight percent sulfuric acid solution. The film is subsequently washed and softened with glycerin to produce the final casing product. The object of this cellophane processing is to take cellulose in one form (a thermal-set polymer fiber) and convert it into a usable porous film. The current estimate is that 50 percent of the carbon disulfide used in the cellulose casing process is converted into pollutants and by-products of the various forms.
The standard thermoplastic bag is not porous. Casing manufacturers have tried to stretch the thermoplastic film to produce the necessary porosity, however this effort has so far been largely unsuccessful.
Casing manufacturers have developed thermoplastic casings which allow the migration of water vapor and smoke. However, these thermoplastic casings have not gained wide acceptance because of expense, poor porosity and poor migration of flavor and color modifiers. These enhancements can, of course, be accomplished in a separate processing step(s), e.g. by placing the processed food in a smoke house. However, this requires additional processing steps.
U.S. Pat. No. 5,374,457 provides a detailed description of thermoplastic casings which contain additives which impart taste or modify the appearance of processed food. In one instance, this reference teaches that melt blending glycerin (liquid smoke) with silica and EVA resin was not a successful combination.
It would be desirable to provide a thermoplastic casing which can be used as an enclosure for processing enclosed food and also as a vehicle for simultaneously transferring modifiers to the food surface during processing.
The casing embodying the invention is based on a process which does not require chemical reactions to produce the casing. The casing is xe2x80x9cenvironmentally cleanxe2x80x9d, has a long shelf life, is not susceptible to bacteria and mold attack as are prior art casings and the casing can also be recycled. The casing can be used as packaging for foods including but not limited to meats, whole or processed, and other produce, vegetables, dairy products, carbohydrates, etcetera.
Broadly the invention comprises combining a food grade thermoplastic with a porosity modifier. The porosity modifier is mixed with the thermoplastic to form a single phase polymer mixture. The mixture is extruded as a sleeve. When the mixture is cooled, the two components undergo phase separation. The porosity modifier defines a network of interconnected interstices (pores or channels) in the thermoplastic which allow a permeate to permeate across (through) the casing and/or allow additives embedded in the casing to flow from the casing to a food product. The interstices are in the range of 0.002 to 1 micron. Prior to use as a casing, the porosity modifier can be either extracted from the thermoplastic or remain in situ.
The casing has a permeability to water vapor of between 1 to 1500 gms/m2/min (standard). The presence, in whole or in part, of the porosity modifier in the casing, will affect the flow of additives into the food. However, when a porosity modifier is present in the casing, it is selected to ensure water vapor and the selected additives will permeate through or from the casing to the food.
In a preferred embodiment of the invention, an inorganic filler is used to impart structural integrity to the casing.
In a preferred embodiment, Nylon is melted and mixed with glycerin and silica to form a single phase polymeric mixture. The mixture is then extruded through a circular die to form a tubular casing. The Nylon and glycerin go through a phase separation process when the mixture cools from the melt temperature to room temperature. The glycerin forms a network of interconnected interstices in the casing. During cooking of a meat product in the casing, the water vapor permeates through the Nylon casing because the water is soluble in the glycerin channels. Smoke can also be transported through the casing based on the same mechanism. The Nylon casing can be water washed to extract the glycerin in whole or in part. The casing derives the needed casing porosity based on dynamics of the Nylon/glycerin (polymer/plasticizer) interaction.
In an alternative embodiment, polypropylene is mixed with soybean oil and silica. The soybean oil can be extracted from the casing in a hexane bath. This can improve the diffusion of the water vapor and additives.
Thermoplastics that can be used are selected from the group consisting of polyethylene, polypropylene, polyvinylidene chloride, polyamide, polystyrene, polyethylene terephthalate, polyvinyl polymer, ethylene vinyl polymer, polycarbonate or polybutene. The amount of thermoplastic in the casing is in a range of approximately 5 to 95% by weight based on the total weight of the casing, preferably 15 to 75%. These polymers have a molecular weight in a range of approximately 10,000 to 5,000,000, preferably 11,000 to 500,000, a density in a range of approximately 0.8 to 1.8, preferably 0.89 to 1.20, a melting point in a range of approximately 100 and 300xc2x0 C., preferably 115 to 265xc2x0 C., a tensile modulus in a range of approximately 25,000 to 500,000 psi, preferably 140,000 to 410,000, a Rockwell hardness in a range of approximately R30 to R130 and from M60 to M-130, and a glass transitional temperature in a range of approximately xe2x88x92130 to 150xc2x0 C.
As will be well understood by one skilled in the art, the polymer may be cross linkable, such as by irradiation, to improve the strength of the film casing. Additives such as anti-oxidants, slip agents, blocking agents and peeling aids can be used to enhance processing of the casing and/or the physical properties of the final product. Antioxidants and antimycotic agents can be used to improve the shelf life of the food products.
The casing can be produced by conventional extrusion or blown film processes. The casing can be multi-layered, e.g. a porous nylon film on top of a porous polypropylene film. The interstices in the casing are in a range of approximately 0.002 to 1 micron, preferably 0.005 to 0.2 micron, the casing can have a thickness in a range of approximately 0.5-15 mils, preferably 1 to 5 and a permeability to water vapor in a range of approximately 1 to 1500 gms/m2/min, preferably 100 to 1000.
Porosity modifiers that can be used are selected from the group consisting of soybean oil, peanut oil, corn oil, glycerin, polyethylene glycol 400, monolaurate, mineral oil or surfactants including polyoxyethylene (POE) 20, sorbitan monostearate (TWEEN 20), POE 80, sorbitan monooleate (MAZ 80), glycerol monooleate. The amount of the porosity modifier in the casing is in a range of approximately 95 to 5% by weight based on the total weight of the casing, preferably 70 to 30% by weight. The porosity modifiers have a density in a range of approximately 0.8 to 1.3.
Inorganic fillers that can be used are selected from the group consisting of silica (SiO2), talc (Mg2SiO4), aluminum oxide, hydrated alumina, titanium oxide, zirconium oxide, sodium silicate, silicate, sodium chloride, calcium, calcium carbonate, clay and calcined clay. The amount of the inorganic filler in the food casing is in a range of approximately 0 to 85% by weight based on total weight of the casing, preferably 0 to 40%. The inorganic filler has a particle size in a range of approximately 1 to 25 microns, preferably 2 to 10 and a density in a range of approximately 1.4 to 5.7, preferably 2.0 to 2.7.
Flavorants and/or fragrances can be added to the casing and comprise liquid smoke extract, vanilla extract, annatto extract, food spices and other food flavor and fragrance extracts. The amount of flavorant and/or fragrances in the casing can be in a range of approximately 1 to 80% by weight, preferably 1 to 40% based on the total weight of the casing. Liquid smoke extract can be neutralized to minimize the oxidation on the thermoplastic casing.
The colorants that can be added to the casing comprise caramel, food dyes, brown sugar and any of the food colorants that are FDA approved for food contact or use. The amount of colorant in the casing can be in a range of approximately 1 to 80% by weight, preferably 1 to 40% based on the total weight of the casing.
A flavorant, fragrance and/or colorant enhanced casing can be laminated or extruded onto a non-porous, non-flavored, non-fragrance and non-colored packaging film. A porous thermoplastic casing can be laminated or extruded onto a non-woven web to improve the strength of the casing.
The preferred method of the invention is to melt the thermoplastic and mix it with silica, colorant, flavorant (liquid smoke) and porosity modifier to form a mixture. The mixture forms a single phase material in an extruder. The mixture is then extruded through a circular die to form the tubular casing (or it can be extruded through a sheet die to form a film. The two edges of the film can be heat sealed together to form a tubular casing). The thermoplastic and the plasticizer go through a phase separation process during cooling. The porosity additive and liquid flavorant form the interstices in the casing. During the cooking of the meat product, the smoke and/or flavor is transported from the casing onto the meat product.
An alternative method to produce a flavorant, fragranced or colored thermoplastic casing is to extrude a casing with the porosity modifier. The porosity modifier can be removed by extraction. The flavorant, fragrance and/or colorant can be absorbed into the casing by coating. The flavorant, fragrance and/or colorant will occupy the interstices of the casing.
The mixing of the thermoplastic, the filler and the porosity modifier is usually accomplished by a xe2x80x9ctwin screwxe2x80x9d extruder. Pellets are produced from the twin screw extruder. The pellets are then be fed to a single screw extruder to produce the final casing. Standard blown film method or the xe2x80x9cdouble bubblexe2x80x9d technique may be used to produce the tubular casing.
The end use of the casing will determine what strength and porosity is required based on how the food is to be processed in the casing. Also, the porosity will be affected by the choice of colorants and/or flavorings. The porosity modifiers may remain in the casing, be partly extracted or fully extracted.
In the following non-limiting examples, silica or talc is premixed with a porosity modifier (generally one part silica to two parts modifier) in a xe2x80x9cdry-blendxe2x80x9d mixer. The silica/modifier mixture is then fed to an extruder along with the thermoplastic and additional porosity modifier. The materials are melted into a single phase molten mixture. The casing material is formed by extruding the mixture either as pellets or a flat film.